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<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=993001018-01022008>Mario,</SPAN></FONT></DIV>
<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=993001018-01022008></SPAN></FONT> </DIV>
<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=993001018-01022008>The selection of components here is not critical. The
10K resistor is used to pull up the gate of the N channel MOSFET because the 555
output is an open collector configuration. (It only pulls down.) The gate of the
MOSFET needs 4 volts or so to overcome the threshold voltage. Since it is a
MOSFET, the gate really does not draw current (except on power on, but the
capacitance is very low and the switching speed is not critical.) Therefore
there is no need for a lower value resistor. 10K and 4.7K are just standard
values I typically use for pullups. </SPAN></FONT></DIV>
<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=993001018-01022008></SPAN></FONT> </DIV>
<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=993001018-01022008>You could also use a P-Channel MOSFET here, although
they typically have a higher on resistance than N-Channels.</SPAN></FONT></DIV>
<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=993001018-01022008></SPAN></FONT> </DIV>
<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=993001018-01022008>As far as the MOSFET selection goes, this is just one
that I like. It has very low on resistance compared with others and favorable
characteristics for this application. (Plus I have a big drawer of
them.)</SPAN></FONT></DIV>
<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=993001018-01022008></SPAN></FONT> </DIV>
<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=993001018-01022008>There are many sources on the internet about the 555
device. I like "The 555 cookbook" which I have in my
collection...</SPAN></FONT></DIV>
<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=993001018-01022008></SPAN></FONT> </DIV>
<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=993001018-01022008>Good Luck,</SPAN></FONT></DIV>
<DIV><SPAN lang=en-us><FONT face="Arial Black" color=#800000 size=2>Rick J.
Bickle</FONT></SPAN> <BR></DIV>
<DIV> </DIV><BR>
<DIV class=OutlookMessageHeader lang=en-us dir=ltr align=left>
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<FONT face=Tahoma size=2><B>From:</B> dprglist-bounces@dprg.org
[mailto:dprglist-bounces@dprg.org] <B>On Behalf Of </B>Mario Marín
Richmond<BR><B>Sent:</B> Monday, January 28, 2008 9:52 AM<BR><B>To:</B>
dprglist@dprg.org<BR><B>Subject:</B> [DPRG] Question about "A Simple PWM Circuit
Based on the 555 Timer"<BR></FONT><BR></DIV>
<DIV></DIV>Hi, my name is Mario and I'm a Power Systems Engineering Student from
Costa Rica University. I was searching in the web for any simple project to
execute for an electronics course and I found your article on the PWM for DC
motors. I would like to know a couple of details about it, such as the values of
the external components and some theorical doubts. Specifically, I would like to
know why the circuit uses the 10K resistance and the IRFZ46N?? I understand that
it must be a N-channel MOSFET, but I must justify why I (in this case you)
picked this component, and if there is any other suitable element that can be
used instead of it. About the resistance, which is its function on the
PWModulator. I would also like to know if there is any bibliographical resource
that may help me create a Theorical and Conceptual Base, explaining the main
concepts involved in PWM. Thanks for publishing the project and thanks in
advance for any help given.<BR><BR><BR><BR>PD: sorry if my english wasn't very
well. <BR><BR>Mario Marin Richmond<BR>Universidad de Costa
Rica.<BR></BODY></HTML>